The present invention relates to a floating apparatus for supporting an offshore platform. The apparatus of the invention includes a plurality of vertical columns attached to a submerged horizontal water entrapment plate on their lower end, and to a deck which supports minimum offshore facilities for the production of hydrocarbons offshore on their upper end.
More particularly, the present invention relates to a floating structure comprising a plurality of vertical columns connected to a horizontal water entrapment plate, the said plate covering the space between the columns and extending outwardly from the lower end of each column such as to form a section of a polygon or circle. In another aspect, the present invention relates to methods for supporting minimum facilities required for the production of offshore hydrocarbon reservoirs from marginal fields.
With increasing exploration activities from offshore basins, such as the Gulf of Mexico, numerous discoveries of relatively small hydrocarbon accumulations have taken place. Many of these fields do not contain sufficiently large amount of oil or gas to justify the expenses of a stand-alone field development, such as a production platform and pipeline infrastructure. In many instances, however, these fields can be produced using subsea-tiebacks to existing infrastructure. These include a subsea wellhead and a flowline to an existing production platform for example.
Serious limitations are expected with longer subsea tie-back, such as plugging of the line due to a decrease in pressure and temperature along the flowline. Conventional remedial measures include injection of chemicals to prevent formation of hydrates. Such chemicals can be transported from the host platform to the subsea wellhead in an umbilical, and can be injected into the flowline at the wellhead. The umbilical can also be used to control the subsea wellhead. The cost of such umbilical is typically very large, and economics of a subsea tie-back is often threatened by the excessive umbilical cost for tie-back distances greater than 20 miles. An alternative development scenario consists of providing a minimum offshore platform near the wellhead with remote control from the host platform and injection of chemicals stored on the minimum offshore platform via a short umbilical connected to the subsea wellhead.
In some cases, where multiphase hydrocarbon flow is expected, the tie-back distance is further limited because of flow assurance issues. Current technological developments are aimed at providing subsea separation facilities to allow hydrocarbons to flow over a greater distance. Such subsea facilities may require additional surface facilities such as power generation and complex control capability.
Similarly, equipment such as subsea pumps may be required to assist flow assurance over the tie-back length. Such pump require power which can be provided by a surface facility located above the pump.
Other technological solutions provided to the flow assurance problem for extended tie-back include electrically heated flowline, which may be heated either continuously or before start-up. The power required to heat the flowline may be produced by a generator located on minimum offshore facilities floating above the flowline.
Current technologies allow certain processing operations to be performed using much smaller equipment than traditional technologies. A minimum offshore platform could therefore be used to perform operations currently conducted on much larger platforms. This could further extend the distance over which hydrocarbon can be transported allowing them in cases to reach the shore directly for further processing.
It is envisioned that future technologies such as fuel cell conversions could be conducted on minimum offshore facilities and power could be shipped via an electrical cable back to shore.
A minimum offshore platform can also be used to perform basic maintenance workover on the wellhead. This saves the high cost of mobilization of a vessel suitable for typical workover operation.
Therefore, there is a need for minimum offshore platform in order to reduce the cost of development of marginal fields so as to make them profitable.
The apparatus described in U.S. patent application 20020044838 filed Feb. 28, 2001 provides a support for minimum offshore facilities, however due to its shape this apparatus suffers from excessive wave induced motion which makes access difficult and dangerous in inclement weather. In addition the motion characteristics result in fatigue of the umbilical or risers connected to it. Due to its motion, it is not possible to land on this facility with a helicopter.
Other platforms, commonly referred to as semi-submersible platforms, have been developed to perform a number of activities related to exploration of and production from hydrocarbon reservoirs. Because of their design including generally rectangular or cylindrical pontoons, their size must be very large, most often in excess of 20,000 tons displacement, in order to provide sufficient stability during extreme weather events. These platforms can therefore carry a large payload, in excess of several thousands tons, but consequently their cost is high, and because of their large size, the required mooring system is also very large and costly.
Thus, in spite of advancements in the art, there still exists a need for a low cost offshore platform to support relatively small payloads for the development of marginal offshore fields, which do not suffer from the disadvantages of the prior art apparatuses.